Lithium compounds are commonly used as initiators for anionic polymerizations. Such organolithium initiators can be employed in synthesizing a wide variety of rubbery polymers. For instance, organolithium initiators can be used to initiate the anionic polymerization of diolefin monomers, such as 1,3-butadiene and isoprene, into rubbery polymers. Vinyl aromatic monomers can, of course, also be copolymerized into such polymers. Some specific examples of rubbery polymers that can be synthesized using organolithium compounds as initiators include polybutadiene, polyisoprene, styrene-butadiene rubber (SBR), styrene-isoprene rubber, and styrene-isoprene-butadiene rubber (SIBR).
The organolithium compounds that can be used to initiate such anionic polymerizations can be either a specific organomonolithium compound or it can be a multifunctional type of initiator. In commercial applications monolithium compounds are normally used because they are available as pure compounds that are soluble in organic solvents. Multifunctional organolithium compounds are not necessarily specific compounds but rather represent reproducible compositions of regulable functionality. Many of such multifunctional organolithium compounds must be stored under refrigeration before being used.
U.S. Pat. No. 5,981,639 explains that multifunctional initiators used to initiate anionic polymerizations include those prepared by reacting an organomonolithium compounded with a multivinylphosphine or with a multivinylsilane, such a reaction preferably being conducted in an inert diluent such as a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound. The reaction between the multivinylsilane or multivinylphosphine and the organomonolithium compound can result in a precipitate which can be solubilized if desired, by adding a solubilizing monomer such as a conjugated diene or monovinyl aromatic compound, after reaction of the primary components. Alternatively, the reaction can be conducted in the presence of a minor amount of the solubilizing monomer. The relative amounts of the organomonolithium compound and the multivinylsilane or the multivinylphosphine preferably should be in the range of about 0.33 to 4 moles of organomonolithium compound per mole of vinyl groups present in the multivinylsilane or multivinylphosphine employed.
U.S. Pat. No. 5,981,639 further notes such multifunctional initiators are commonly used as mixtures of compounds rather than as specific individual compounds. Other multifunctional polymerization initiators can be prepared by utilizing an organomonolithium compound, further together with a multivinylaromatic compound and either a conjugated diene or monovinylaromatic compound or both. These ingredients can be charged initially, usually in the presence of a hydrocarbon or a mixture of a hydrocarbon and a polar organic compound as a diluent. Alternatively, a multifunctional polymerization initiator can be prepared in a two-step process by reacting the organomonolithium compound with a conjugated diene or monovinyl aromatic compound additive and then adding the multivinyl aromatic compound. Any of the conjugated dienes or monovinyl aromatic compounds described can be employed. The ratio of conjugated diene or monovinyl aromatic compound additive employed preferably should be in the range of about 2 to 15 moles of polymerizable compound per mole of organolithium compound. The amount of multivinylaromatic compound employed preferably should be in the range of about 0.05 to 2 moles per mole of organomonolithium compound. Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene, 1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene, 1,3-divinylnaphthalene, 1,8-divinylnaphthalene, 1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl, m-diisopropenyl benzene, p-diisopropenyl benzene, 1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatic hydrocarbons containing up to 18 carbon atoms per molecule are preferred, particularly divinylbenzene as either the ortho, meta or para isomer and commercial divinylbenzene, which is a mixture of the three isomers, and other compounds, such as the ethylstyrenes, also is quite satisfactory.
U.S. Pat. No. 4,196,154 discloses organic liquid soluble multifunctional lithium containing initiators are prepared by reacting an organo lithium compound with an organic compound containing at least one group of the configuration 1,3-bis(1-phenylethenyl)benzene. U.S. Pat. No. 4,196,154 reports that such initiators can be prepared in the absence of polar solvents and are very desirable for the polymerization of dienes such as butadiene to a desirable 1,4 configuration.